Explain how the fluid mosaic model accounts for the selective permeability of the plasma membrane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The phospholipid bilayer has a hydrophobic core that blocks polar or charged molecules such as ions and glucose, preventing their passage.
Small non‑polar, lipid‑soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely through this core by simple diffusion.
Integral or transmembrane proteins – channels or carriers – span the bilayer and provide specific routes for polar or charged substances to cross.
Because the membrane is a fluid mosaic, proteins can be distributed unevenly; different regions can have different protein compositions, giving the membrane variable permeability across the cell surface.
Small non‑polar, lipid‑soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely through this core by simple diffusion.
Integral or transmembrane proteins – channels or carriers – span the bilayer and provide specific routes for polar or charged substances to cross.
Because the membrane is a fluid mosaic, proteins can be distributed unevenly; different regions can have different protein compositions, giving the membrane variable permeability across the cell surface.
Examiner tips
- Use the phrase ‘hydrophobic core blocks polar/charged molecules’ to hit the first point. Include examples of non‑polar molecules and of channel/carrier proteins. Show how the fluid mosaic allows regional variation in permeability.
Mark scheme (4 marks)
- The phospholipid bilayer forms a hydrophobic core that prevents the passage of polar/charged molecules (e.g. ions, glucose)
- Small non-polar / lipid-soluble molecules (e.g. O₂, CO₂, steroid hormones) can pass freely through the phospholipid bilayer by simple diffusion
- Integral/transmembrane channel or carrier proteins span the bilayer and allow specific polar/charged substances to cross
- The mosaic/fluid arrangement allows proteins to be distributed according to the cell's transport needs, so different membrane regions can have different permeabilities / the composition of the membrane can vary
Key terms in this question
fluid mosaic model · selective permeability
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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